It is kown that telephone transmission generally takes place on two-wire lines for short distance connections, while long-distance connections are effected over 4-wire lines.
Between the two kinds of lines the so-called "hybrids" are inserted, i.e. differential transformers with three terminations available, one being connected to the two-wire line and each one of the remaining two being connected to a monodirectional circuit of the 4-wire lines.
Under ideal conditions, the energy present on the 4-wire line ought to be exactly subdivided between the two-wire line and the balancing impendance. However, actually the hybrid coupling is never perfect, and consequently reflections or echoes arise.
More particularly, a part of the signal coming from one of the monodirectional circuits of 4-wire lines is sent back towards the source along the other monodirectional circuit, so that the subscriber, while transmitting, hears again what he has previously transmitted with a delay equal to the time taken by the signal to cover the path to and from the remote hybrid. It has been proved that echo effects are annoying only for delays exceeding a certain value, e.g. 50 ms, and increase with the delay, making the speech practically unintelligible.
In certain cases the echo signal characteristics are non-stationary, that is why after a certain time the estimate effected at the beginning of the iterative procedure is no longer valid.
Non-stationarity causes can depend on the connection among the subscribers, and hence negligeable for the echo cancelling during a given communication, or can take place during a communication. In the two-wire peripheral part of the telephone network translated-band FDM transmission systems (frequency division multiplex) are widely used: this transmission technique gives rise to non-stationarity phenomena during a communication.
According to this type of modulation, one only of the two transmission directions undergoes a further modulation-demodulation process at both ends of the transmission line.
This is effected either to allow, as in the case of 12-channel FDM systems, two-wire bidirectional transmission or, in systems with a higher capacity with transmission onsymmetrical-pair cables, to avoid cross-talk.
Since modulation-demodulation frequencies are locally generated, they can differ from one another, and a voice-band frequency of value f.sub.o before the modulator may be returned by the demodulator with a frequency-shift .DELTA.F.
In such a case a phase-shift exists between modulators-demodulators (the so called phase-roll) which causes a shift of all the frequency components of the echo signal with respect to the main signal.
In addition it should be noted that in the present network multiple-echo cases are possible. When the connection presents a number of 4-wire trunks interspaced by 2-wire trunks, and in same or all the 4-wire trunks translated-band FDM systems are present, overall echo is composed of a plurality of significant reflection components, each affected by a different phase-roll.
A further contribution to the phase-roll can be given by band-inversion line-repeaters of 12-channel systems; in this case in fact a modulation and a demodulation have to be effected, with relevant locally generated frequencies to allow different amplification in the two transmission directions.
CCITT recommends (Rec. G311) a .DELTA.F between subscribers lower than 2 Hz in both directions.
In such a case the maximum .DELTA.f for the canceller is 4 Hz.
Echo cancellers known in the art generally consist of:
a transversal digital filter forming the echo channel model; PA0 a circuit for computing the digital filter coefficients; PA0 a circuit for computing the updating rate of the coefficient values; PA0 a double talking detector which inhibits the updating process when the near talker is speaking; PA0 a device called center-clipper for silencing the residual echo.
The echo-estimate has been carried out by copying the pulse-response of the equivalent echo channel, i.e. by computing the convolution between the samples of the digital signal at the input of the far talker side and the transversal filter coefficients; the signal outgoing from the far-talker side, i.e. the residual echo, is obtained by subtracting the echo estimate from the signal at the input of the near-talker side.
The method generally used to compute filter coefficients is the minimization of the residual-echo power, by estimating the power itself either sample by sample or on a sequence of samples. To carry out this minimization an algorithm of the adaptive type is used, by which the coefficients are iteratively computed by adding to the present coefficient value a correction proportional to the correlation between the sample corresponding to the coefficient and the residual-echo sample.
The pulse-response of the echo channel basically consists of a variable pure delay, the so-called flat-delay, due to the variable propagation velocity of the signal in the transmissive medium, and to the delay introduced by the transmission devices; said flat-delay is followed by the significant portion of the pulse response.
Said known cancellers cannot be used for connections which are subjected to phase-roll and which consequently have characteristics which rapidly change in time. In fact said known cancellers do not generally present an updating rate sufficient to cope with channel-characteristic variations due to the phase-roll.
The implementation of an echo-canceller for systems with time-varying characteristics is already known in the art and is described in U.S. Pat. No. 4,072,830, inventors R. D. Gitlin, J. S. Thompson, in the name of Bell Telephone Laboratories.
This implementation provides an SSB modulator in series with the transversal filter for the phase-roll recovery, i.e. the input samples at the far-talker end, before entering the echo-canceller, are modulated with a Weaver modulator, with a modulating frequency estimated starting from the error signal with known adaptive techniques.
Such a structure is usable only if the whole echo channel is affected by the same frequency shift, i.e. by the same phase-roll amount.
On the contrary it is not usable in the case of multiple echo, where a plurality of different phase-rolls are possible.